1. Field of the Invention
The present invention relates to a multiple-input multiple-output (MIMO) mobile communication system, and more particularly, to a method for efficiently transmitting and receiving signals in an open-loop spatial multiplexing mode.
2. Discussion of the Related Art
With the popularization of information communication services, the emergence of various multimedia services, and the provision of high-quality services, demand for rapid wireless communication service has increased. To actively cope with such demand, first of all the capacity of a communication system should be increased. To increase communication capacity in a wireless communication environment, there can be considered a method for newly searching available frequency bands and a method for increasing efficiency for limited resources. As to the latter method, multiple transmit and receive antenna techniques have recently drawn attention and have been actively developed. The multiple transmit and receive antenna technique obtains a diversity gain by equipping a transmitter and a receiver with a plurality of antennas to additionally ensure a spatial region for utilizing resources, or increases transmission capacity by transmitting data in parallel through the respective antennas.
A MIMO system using an orthogonal frequency division multiplexing (OFDM) among the multiple transmit and receive antenna techniques will now be described.
FIG. 1 illustrates a general structure of a multiple transmit and receive antenna system using OFDM.
In a transmitting side, a channel encoder 101 adds redundancy bits to transmission data bits to reduce an influence of a channel or noise, and a mapper 103 converts data bit information into data symbol information. A serial-to-parallel converter 105 parallelizes the data symbol information to carry data symbols on a plurality of subcarriers. A multiple antenna encoder 107 converts the parallelized data symbols into time-space signals. In a receiving side, a multiple antenna decoder 109, a parallel-to-serial converter 111, a demapper 113, and a channel decoder 115 respectively perform the reverse functions of the functions performed in the multiple antenna encoder 107, the serial-to-parallel converter 105, the mapper 103, and the channel encoder 101 of the transmitting side.
The multiple antenna OFDM system requires various techniques to improve the reliability of data transmission. A space-time coding (STC) scheme and a cyclic delay diversity (CDD) scheme are used to raise a spatial diversity gain. A beam forming scheme and a precoding scheme are used to increase a signal-to-noise ratio (SNR). The STC and CDD schemes are mainly used to improve transmission reliability of an open-loop system which can not use feedback information in a transmitting side. The beam forming and precoding schemes are used to maximize the SNR through corresponding feedback information in a closed-loop system which is capable of using feedback information in the transmitting side.
In the above-described techniques, the CDD scheme for increasing the spatial diversity gain and the precoding scheme for raising the SNR will now be described.
The CDD scheme causes all antennas to transmit signals with different delays or different sizes in transmitting OFDM signals in a system having multiple transmit antennas, so that a receiving side obtains a frequency diversity gain.
FIG. 2 illustrates a structure of a transmitting side of a multiple antenna system using a CDD scheme.
While OFDM symbols are separated through a serial-to-parallel converter and a multiple antenna encoder and transmitted to each antenna, a cyclic prefix (CP) for preventing interference between channels is added and then transmitted to a receiving side. In this case, a data sequence transmitted to the first antenna is transmitted to the receiving side without delay, and data sequences transmitted to the next antennas are cyclically delayed by a predetermined sample compared with the preceding antennas.
Meanwhile, if the CDD scheme is performed in a frequency domain, the cyclic delay may be expressed as a multiplication of phase sequences.
FIG. 3 illustrates a method for performing the CDD scheme shown in FIG. 2 in a frequency domain.
As shown in FIG. 3, data sequences in a frequency domain are multiplied by phase sequences (phase sequence 1 to phase sequence M) which are differently set according to antennas, and thereafter, inverse fast Fourier transform (IFFT) is performed to transmit the data sequences to a receiving side. This method is referred to as a phase-shift diversity scheme.
The phase-shift diversity scheme may convert a flat fading channel into a frequency selective channel, and obtain a frequency diversity gain through a channel code or a multi-user diversity gain through frequency selective scheduling.
Meanwhile, the precoding scheme includes a codebook based precoding method used when feedback information is finite in a closed-loop system and a method for performing feedback upon quantization of channel information. Codebook based precoding refers to obtaining an SNR gain by feeding back an index of a precoding matrix, which is previously known by transmitting and receiving sides, to the transmitting side.
FIG. 4 illustrates a structure of transmitting and receiving sides of a multiple antenna system using codebook based precoding.
The transmitting side and receiving side respectively include finite precoding matrixes P1 to PL. The receiving side feeds back an optimal precoding matrix index I to the transmitting side using channel information. The transmitting side may apply a precoding matrix corresponding to the fed back index to transmission data X1 to XMt.
The above-described phase-shift diversity scheme or the CDD scheme may have different requirements in an open-loop type and a closed-loop type depending on whether the feedback information is demanded. That is, it may be desirable that different precoding matrixes be used in an open-loop CDD scheme and a closed-loop CDD scheme.
Under such an assumption it is necessary to definitely specify a method for selecting a proper precoding matrix while acquiring a sufficient frequency diversity gain and simultaneously minimizing the complexity of achievement according to each CDD scheme, and for efficiently transmitting and receiving signals.